Heat sensitive recording materials having a heat sensitive color forming layer which forms color by heating and is formed on a support such as paper, synthetic paper and a plastic film are utilized for thermal printers for various portable terminals, printers of medical images attached to supersonic echo image processing instruments, thermopen recorders for electrocardiograms and analytical instruments, airplane tickets, train tickets and POS labels for merchandises.
The heat sensitive recording materials are produced, in general, as follows: a colorless or slightly colored leuco dye as the color forming substance and a color developer which develops color by reaction with the color forming substance to record data are finely pulverized and dispersed in a solvent; sensitizing substances for enhancing the effects of the color forming substance and the color developer, waxes, surfactants, defoaming agents and inorganic pigments are added, where necessary; binders such as resins soluble in water are added; and the resultant mixture is applied to a support such as paper to form a coating layer and dried to form a color forming layer. As the color developer which develops color by reaction with the color forming substance, various phenol compounds are widely used.
For applications requiring a great sensitivity to heat, a sensitizing substance which lowers the temperature of the color formation and enhances the effects of the color forming substance and the color developer is added. As the sensitizing substance, for example, paraffin wax, fatty acid amides and aromatic esters are used. Various properties, such as the excellent whiteness without fog on the background, an excellent color forming property to form color with a small energy and an excellent property for storage of the obtained images, are required for the heat sensitive recording material. In particular, the color developing substance is an important factor for the requirements for the heat sensitive recording material. Even when the color forming substance and the sensitizer exhibit excellent properties, occasionally, no excellent recording materials can be obtained due to a color developer exhibiting poor properties.
The present invention has an object of providing a color developer for heat sensitive recording and a heat sensitive recording material which suppress fog on the background and exhibit excellent color forming property and storage property of image portions.